1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an optical pickup device capable of easily realizing recording/reproducing of an optical disc having at least two recording layers on one side thereof.
2. Description of the Prior Art
Information reproducing/recording systems for using optical discs such as laser discs or compact discs have been developed in recent days. Higher data densities are desirable for a variety of reasons, including the ability to make compact discs smaller and to enable the recording of High Definition Television (HDTV) compatible video discs.
One method considered possible for achieving a high data density in an optical data medium was to focus the read laser to an even finer beam diameter, but the maximum practical limit to the numerical aperture (NA) of an objective lens which focuses the laser is approximately 0.6. It would also be possible to obtain a similar result by shortening the laser wavelength, but a wavelength of a practical semiconductor laser is limited for the present.
A second method for achieving a high density is to use V-grooves in the optical disc, such as disclosed in U.S. Pat. No. 4,569,038 issued to Nagashima et al. (Feb. 4, 1986).
The capacity of the data recordable on an optical disc using conventional technologies is determinable. For an optical disk housed in a 3.5 inch cassette case, the disc diameter is approximately 86 mm, providing a data storage area with a radius of 25 mm to 41 mm. It is assumed that a 670 nm wavelength laser is used with a 0.55 NA objective lens. With a 0.8 m track pitch (1.6 .mu.m V-groove pitch) and 0.47 .mu.m bit length, the unformatted single-side capacity is 1.1 Gbytes. Using the standard CD format, a single-side formatted capacity of 550 Mb, the same as a 12 .mu.cm CD, can be obtained.
If information conventionally provided as printed material is to be provided in the future on a CD-ROM or other optical data medium, it will also be necessary and convenient if the user is able to write to the medium to record notes as is done currently with paper media, instead of only being able to read the information. One method which makes this possible divides the disc into a read/writable areas, wherein a recording film is formed on the inside portion of the disc, and a read-only data area comprising a reflective metal film on the outside portion of the disc. However, this method reduces the read-only data area, and therefore limits the amount of information distributable on a single disc.
As thus described, while smaller, higher capacity optical data media, including optical discs and cards, are desirable. To satisfy this requirements, an optical medium having two data storing layers has been developed, and such an optical pickup device is disclosed in U.S. Pat. No. 5,134,604 issued to Nagashima et al.
A conventional optical pickup device will be described in detail with reference to FIG. 1 below.
FIG. 1 is a schematic view of the conventional optical pickup device. A light source 10 is a laser diode for generating a laser beam for recording/reproducing information on/from an optical disc D. The laser beam from laser diode 10 is converted into parallel rays through a collimator lens 11. The beams converted into the parallel rays by collimator lens 11 sequentially pass through a beam splitter 12, an objective lens 14 and a transparent substrate 16 to be incident to the surface of optical disc D with a beam spot of approximately 1 .mu.m diameter.
Here, beam splitter 12 is provided in such a manner that two right-angled prisms are installed to oppose to each other along respective inclination planes of 45.degree., and a coating layer 13 is formed along the contacting portion of the prisms, thereby securing the property of travelling straight of the incident beam while transmitting some of the incident beam and reflecting the other incident beam at a right angle with respect to the incident beam.
The laser beam being straightly transmitted through beam splitter 12 passes through transparent substrate 16 which is removably arranged between objective lens 14 and optical disc D. The focal distance of the laser beam can be arbitrarily controlled in accordance with the arranged position, refractive index and thickness of transparent substrate 16. Therefore, the position, refractive index and thickness of transparent substrate 16 are properly determined to focus the beam from laser beam source 10 on a nearer data recording plane of optical disc D under the state of eliminating transparent substrate 16, and allow the laser beam to pass through transparent substrate 16 of a predetermined thickness prior to focusing on a farther data recording plane of optical disc D under the state of inserting transparent substrate 16 of the predetermined thickness.
The laser beam reflected at the right angle while passing through beam splitter 12 focuses while passing through a focusing lens installed along an optical path of the reflected light, and the light focused by focusing lens 15 is received into a four or six division light-receiving diode 17. Using an image formed by the beam received into light-receiving diode 17, a positional accuracy, i.e., focusing error and tracking error, of the optical pickup device with respect to optical disc D is detected.
By this operation, the focusing and tracking are controllable along with reproducing the information on the basis of the amount of the reflected lights determined by pits of optical disc D.
According to the conventional optical pickup device constructed as above, however, a lot of equipments including a transferring apparatus of the transparent substrate and several sensors are required for inserting and eliminating the transparent substrate. Additionally, a working distance of the objective lens is shortened to make the control of the focusing and tracking a demanding job, thereby degrading quality of the pickup. Furthermore, it involves a technical difficulty since a tolerance below 0.1.degree. should be controlled to insert the transparent substrate.